The macromolecules in mammalian uterine cytosols responsible for the high-affinity and specific binding of steroid hormones, in particular progestins and glucocorticoids, are the object of this study. Working with purified preparations, we intend to study the interactions of steroid receptor complexes with nuclear material, the relationship of these interactions to the biological effects of the hormones and the possible clinical value of receptor studies in the diagnostic and prognostic assessment of endometrial cancer. The uterine cytosols will be fractionated on spheroidal hydroxyapatite columns to separate the progestin-specific receptor from other highaffinity binding proteins. Endometrial cultures and suppression techniques will be used to study the nuclear interactions of receptor complexes and the hormonal effects. The determination of bound steroids will be carried out by means of a DEAE-cellulose filtration assay.